The ultrasound pulse-echo technique (echography) is widely used in medical imaging. This imaging method currently uses an array of transducer elements to transmit a focused beam into the body, and each element then becomes a receiver to collect the echoes. The received echoes from each element are dynamically focused to form an image. Focusing on transmission and reception is performed assuming that the velocity inside the body is uniform, and is usually assumed to be 1540 m/s. Unfortunately, the velocity inside the body is not constant; it varies from 1470 ms.sup.-1 in fat to greater than 1600 ms.sup.-1 in some other tissues, such as collagen. This variation will result in increased side lobes and degraded lateral resolution. It is one of the major difficulties for improving lateral resolution of ultrasound imaging system. Phase aberration caused by velocity variation also influence the accuracy of many other measurements, such as attenuation and blood flow velocity. Amplitude aberrations have also been observed and reported in many works in ultrasound medical imaging, especially for imaging some complex tissue structures like female breast. Amplitude aberrations also influence the quality of images, even though they are not as important as phase aberrations. In some cases, both amplitude and phase aberration corrections are needed to form a good image.
Many methods have been developed to correct aberrations. These prior art methods are reviewed below.
One type of prior art method uses the wavefront from a special target, such as a dominant point target or a specular reflecting plane, to measure the phase-aberration profile. In astronomical imaging, direct-wavefront-measurement method is used to measure phase and amplitude aberrations caused by the atmosphere (R. K. Tyson, "Principle of adaptive optics," Academic Press, ch. 5, 1991). In medical ultrasound imaging, the nearest neighbor cross-correlation algorithm (S. W. Flax and M. O'Donnell, "Phase-aberration correction using signals from point reflectors and diffused scatterers: basic principles," IEEE Trans. Ultrason., Ferroelect., Freq. Cont., vol. 35, no. 6, pp. 758-767, November 1988, and U.S. Pat. No. 4,989,143 by O'Donnell) and the maximum-sharpness algorithm (L. Nock, and G. E. Trahey, "Phase aberration correction in medical ultrasound using speckle brightness as a quality factor," J. Acoust. Soc. Am., vol. 85, no. 5, pp. 1819-1833, May 1989) can be used when there is a dominant point target.
The nearest-neighbor cross-correlation and maximum sharpness algorithms can also uses echoes from randomly distributed scatterers that generate speckle in an image to measure the phase-aberrations. In the nearest-neighbor cross-correlation method, a focused beam is transmitted and the phase-aberration profile is derived from the cross-correlation measurements between neighboring elements. An iterative method is used to improve the measurement accuracy. The maximum sharpness an iterative phase-correction procedure in which the timing of acoustic signals transmitted and received from individual elements is adjusted to optimize the quality indicator.
Prior art methods which uses the principle of the nearest-neighbor cross-correlation algorithm include U.S. Pat. No. 4,471,785 by Wilson et al., U.S. Pat. No. 4,817,614 by Hassler et al., U.S. Pat. No.5,184,623 by Mallart, 5,172,343 by O'Donnell, U.S. Pat. No. 5,388,461 by Rigby, U.S. Pat. No.5,487,306 and 5,531,117 by Fortes, U.S. Pat. Nos.5,423,318 and 5,566,675 by Li et al.
Prior art methods which uses the principle of the maximum sharpness algorithm include U.S. Pat. No.4,852,577 by Smith et al., U.S. Pat. No. 5,331,964 by Trahey et al., and U.S. Pat. No.5,423,318 by Green.
The Translating Apertures algorithm (W. F. Walker and G. E. Trahey, "Speckle coherence and implications for adaptive imaging," J. Acoust. Soc. Am., vol. 101, no. 4, pp. 1847-1858, April 1997 and U.S. Pat. No. 5,673,699 by Trahey et al) is a modification of the nearest neighbor cross-correlation algorithm. It uses identical effective apertures to collect near-neighbor signals.
The differences between the near-field signal redundancy algorithm described in the present invention, the nearest-neighbor cross-correlation algorithm, and the translating apertures algorithm have been discussed in Yue Li and Robert Gill, "a comparison of matched signals used in three different phase-aberration correction algorithms" 1998 IEEE International Ultrasonics Symposium. One of the differences between the nearest-neighbor cross-correlation algorithm and the algorithm in present invention is that, non-common midpoint signals are included in matched signals collected with the nearest-neighbor cross-correlation algorithm, but they are not included in the matched signals collected with the algorithm described in the present invention. This results in increased similarity between matched signals. The translating apertures algorithm is also different from the near-field signal redundancy algorithm described in the present invention. One of the major differences is that reciprocal signals are always included in matched signals collected with the translating apertures algorithm, but they are not included in the algorithm described in the present invention. It is a disadvantage to include the reciprocal signals in matched signals, since reciprocal signals are not sensitive to phase aberrations. When phase-aberrations exist they decrease the similarity between matched signals and reduce the measurement accuracy.
A method using the signal-redundancy principle to measure the phase-aberration profile has been developed (D. Rachlin, "Direct estimation of aberration delays in pulse-echo image systems," J. Acoust. Soc. Am. vol. 88, no. 1, pp. 191-198, July 1990 and U.S. Pat. No.5,268,876 by Rachlin). Since signal redundancy principle is an approximation for targets in the near field, common-midpoint signals are not identical for targets in the near field. Additional signal processing is required to make it work properly for targets in the near field. In the theoretical analysis, Rachlin has assumed that targets are small and compact (Col. 3, lines 8) located at the focal point so that far field analysis can be used, and proposed to use the whole aperture for transmission in the algorithm to allow extended target distribution (Col. 6, lines 53), and as a result, solving the near-field problem. One disadvantage of transmitting from the whole aperture is that the transmitted beam will be distorted by phase aberrations and it will influence the measurement accuracy. Another disadvantage of transmitting from the whole aperture is that the far-field approximation is only valid in a small depth range around the focal point (if the focus is not already distorted by phase aberrations), therefore only a short signal length can be used for the measurement. The near-field signal redundancy algorithm described in the present invention uses a different method to solve the near-field problem. A technique of dynamic near-field correction applied on common-midpoint signals is proposed to make common-midpoint signals become more similar for targets in the near field. It allows a long period of signals be used for the measurement to increase measurement accuracy. Using small apertures for transmission will reduce the influence of aberration on the measurement accuracy. Other near-field signal redundancy algorithms described in the present invention which are not included in Rachlin's method are: the sub-array technique for collecting common-midpoint signals, near-field signal redundancy algorithms for measuring phase aberrations when the transmission and reception aberration profiles are different, near-field signal redundancy algorithms for two-dimensional arrays, near-field signal redundancy algorithms for amplitude-aberration corrections etc.
Common-midpoint signals are also used for aberration measurement in seismic imaging (O. Yilmaz, "Seismic data processing," Society of Exploration Geophysicists, ch. 3, 1987). It uses echoes from a specular reflecting plane which is a special kind of target. The common midpoint signals are not redundant when there is a specular reflecting plane in the near field, because the position of the reflecting point is different for different transmitter or receiver positions. Therefore, the seismic method is not a signal-redundancy method and is fundamentally different from the method proposed in this invention.